1. Field of the Invention
The present invention relates to a process for exchanging a dispersing medium of an aliphatic carboxylic acid slurry of terephthalic acid. More specifically, it relates to a process for obtaining an aqueous slurry of terephthalic acid by exchanging, for example, the aliphatic acid, which is the dispersing medium in an aliphatic carboxylic acid slurry of terephthalic acid obtained by an oxidation of p-xylene in an aliphatic carboxylic acid dispersing medium, for water.
2. Description of the Related Art
Terephthalic acid is very useful as the starting material for polyester resin, etc., and is usually obtained by an oxidation of p-xylene in an aliphatic carboxylic acid solvent such as acetic acid, and as an aliphatic carboxylic acid slurry from the reaction apparatus. Terephthalic acid is insoluble in solvents such as water, alcohol, ether, chloroform, and acetic acid, and is sublimated without melting by heating to 300.degree. C., and therefore, it is difficult to obtain a high purity terephthalic acid by utilizing conventional methods such as recrystallization or vacuum distillation from an aliphatic carboxylic acid slurry of terephthalic acid discharged from the reaction apparatus.
Accordingly, in the purification of terephthalic acid, a method has been employed in which crude crystals of terephthalic acid are separated from an aliphatic carboxylic acid slurry of terephthalic acid obtained from a reaction apparatus, the crude crystals are dispersed in water to obtain an aqueous slurry of terephthalic acid, and the aqueous slurry is hydrogenated by, for example, contact with hydrogen gas, to improve the solubility of the impurities and remove the impurities by dissolution into the solvent. In the steps of this purification method, since crude crystals of terephthalic acid are in the form of a fine powder and have a very low filtration efficiency, it is difficult to use a separation by filtration method, and thus a separation method such as centrifugation is usually adopted.
Nevertheless, to effectively separate crude crystals from the aliphatic carboxylic acid slurry of terephthalic acid by a centrifugation device, the centrifugation must be carried out at least two or three times, or more, and further, the crude crystals obtained by centrifugation must be dried to enable a reduction of the terephthalic acid crystals to a powder. Accordingly, a plurality of centrifugation devices, a drying device, and a storage for the particles are required, and therefore, the ratio of equipment cost to production cost becomes very high. Further, a problem arises in that the separation of crystals as described above is very cumbersome. Also, when producing an aqueous slurry by a dispersion of the terephthalic acid obtained as crude crystals in water, a problem arises in that the preparation of the aqueous slurry, such as the stirring of a mixture of the crude crystals and water at a high speed, is very cumbersome.
Japanese Unexamined Patent Publication (Kokai) No. 57-53431 discloses a washing process which is primarily characterized by washing crude terephthalic acid in a multi-stage washing column provided with partitioning plates having a plurality of holes arranged therein. This washing entails specifically a process in which terephthalic acid particles are washed when a terephthalic acid slurry in an aqueous mother liquor is introduced into the multi-stage washing column at the top thereof and water is introduced at the bottom thereof to effect a counter current contact therebetween, whereby terephthalic acid particles are sedimentated, and the dispersing medium constituting the terephthalic acid slurry and the washing solvent are both the same liquid, i.e., water, although some differences in the purity of both may exist.